Campbell Biology 10e, in discussing the functions of introns, writes:
The presence of introns in a gene may facilitate the evolution of new and potentially beneficial proteins as a result of a process known as exon shuffling. Introns increase the probability of crossing over between the exons of alleles of a gene—simply by providing more terrain for crossovers without interrupting coding sequences. This might result in new combinations of exons and proteins with altered structure and function. We can also imagine the occasional mixing and matching of exons between completely different (nonallelic) genes. Exon shuffling of either sort could lead to new proteins with novel combinations of functions. While most of the shuffling would result in nonbeneficial changes, occasionally a beneficial variant might arise.
Hitherto, I had perceived crossing over as the shuffling of alleles between two homologous chromosomes, not of parts of alleles. That is, breaks in the DNA during synapsis would occur in between loci, not within them. Am I wrong to think that? My books tell me that, with regards to evolution, sources of genetic variation include mutations and genetic recombination. Genetic recombination includes random fertilization, independent assortment of homologs, and crossing over, yet it is only mutations that can produce new genetic material. If exons were crossed over, then new alleles would be formed, would they not? Is this a type of mutation? (I do not recall studying such a specific case of mutations.)